This invention relates to a differential comparator circuit and, in particular, to a differential comparator circuit which includes compensation for differences in the electrical characteristics of transistors forming the differential comparator.
Differential amplifiers are widely used as comparators. For example, the input stage of a standard differential amplifier comparator formed using insulated-gate field-effect transistors (IGFETs) normally includes first and second IGFETs having their source electrodes connected in common to a common current source and with an input signal applied to the gate of the first IGFET and a reference signal applied to the gate of the second IGFET. A problem arises in the use of the differential amplifier as a comparator if the responses of the first and second IGFETS are not equal and symmetrical. This is the case, for example, where the threshold voltages (i.e. the gate-to-source voltages, VGS, at low source-to-drain current) of the first and second IGFETs are different for the same bias conditions. Depending on which transistor has the larger threshold voltage, the input signal will appear to be greater or smaller than it actually is, resulting in an error.
This problem is very pronounced when the IGFETs used to form the comparator are thin film transistors (TFTs) made of amorphous silicon. The threshold voltages of TFTs or IGFETs made of amorphous silicon tend to be non-uniform due to processing and other factors and to shift or drift during operation when subjected to different stress potentials. As a result of these characteristics, amorphous silicon transistors would normally not be used to form a differential comparator. However, there are certain system applications where it is highly desirable and/or necessary to use such transistors. One of these applications relates to circuitry for driving liquid crystal displays. Amorphous silicon is a desirable technology for fabricating liquid crystal displays because this material can be fabricated at low temperatures which permits the use of standard, readily available, and inexpensive substrate materials.
The liquid crystal displays are normally composed of a matrix of pixels arranged in rows and columns. These pixels have to be driven by row and column drivers which are in turn driven by switching and control circuits. For purpose of economy of space and to reduce fabrication cost, it is preferable to simultaneously fabricate the control and switching circuitry driving the rows and columns of the liquid crystal display elements using the same technology used to form the liquid crystal display. Furthermore, it is desirable to form the control and switching circuitry around the periphery of the liquid crystal display at the same time as the liquid crystal display is formed. Therefore, there are compelling economic reasons (among others) for employing amorphous silicon transistors in the design of the control and switching circuitry including any comparator circuits.
As detailed below, applicant's invention is directed to a comparator circuit which includes compensation for differences and shifts in the threshold levels of the transistors forming the comparator. The compensation scheme of the invention enables an accurate comparator to be formed using transistors having non-uniform and/or varying characteristics, and may be formed using any known technology and may include IGFETs and/or bipolar transistors.